In electrical systems power conservation is always an important factor. In some electrical systems power conservation may even be a critical factor. Mobile electronic devices such as wireless cellular telephones usually operate on battery power. The battery must be periodically replaced or periodically recharged.
Some electrical system applications such as wireless sensor networks operate at low power and low data rates. Batteries that have a long life (e.g., up to ten years) are usually employed in such applications. In some applications replacing batteries (even long-lived batteries) is not practical. Replacement battery costs, labor costs, and hard-to-access locations may make the use of battery sources of power impractical in some cases. Therefore, systems and methods have been sought that extract (or harvest) electrical power from the environment.
Piezoelectric materials have been used to extract electrical power from the environment. It is well known that piezoelectric materials produce electric charges on portions of their surfaces when they are under mechanical stress. Either compressive stress or tensile stress on a piezoelectric material will cause electrical charges to be generated at the surface of the piezoelectric material.
A mechanical stress that is applied to a piezoelectric material will produce an electric polarization in the piezoelectric material that is proportional to the applied stress. The electric polarization manifests itself as a voltage across the piezoelectric material. It is well known that piezoelectric materials may be used in electromechanical transducers to convert mechanical energy to electrical energy.
In some embodiments a piezoelectric vibrating energy harvester comprises a cantilever structure that supports a piezoelectric film (e.g., a lead-zirconate-titanate (PZT) film). Ambient vibrations in the environment cause the cantilever and piezoelectric film to move back and forth (vibrate). The stress applied to the piezoelectric film causes the piezoelectric film to transform the energy in the ambient vibrations into electrical energy that can be accumulated and stored for later use. This type of piezoelectric power generation provides an alternative power source for operating low power very large scale integration (VLSI) electronic devices.
An example of such a micro-electromechanical vibrating energy harvester that is based on piezoelectric power generation principles is described in United States Patent Application Publication No. 2007/0125176 for a patent application by Yue Liu that was filed on Dec. 2, 2005 and published on Jun. 7, 2007.
FIG. 1 illustrates a prior art piezoelectric vibrating energy harvester 100. As shown in FIG. 1, the harvester 100 comprises a fixed end 110. A first end of a cantilever host beam 120 is fixed to the fixed end 110. The second end of the cantilever host beam 120 is not fixed to any structure and is free to move up and down (i.e., to vibrate) in a vertical direction.
A piezoelectric layer 130 is placed and positioned on top of the cantilever host beam 120 along the length of the cantilever host beam 120. A mass 140 (e.g., a block of metal 140) is placed on top of the piezoelectric layer 130. The mass 140 is preferably placed at the freely vibrating second end of the cantilever host beam 120.
The cantilever host beam 120 and the piezoelectric layer 130 vibrate in response to ambient vibrations that cause the mass 140 to move up and down. The piezoelectric layer 130 transforms the vibrations into electrical energy that appears as alternating current (AC) voltage (designated in FIG. 1 as VOUT) across the piezoelectric layer 130. A first electrical connection 150 connects a first electrical output of the piezoelectric layer 130 to a first input of a rectifier circuit 160. A second electrical connection 170 connects a second electrical output of the piezoelectric layer 130 to a second input of the rectifier circuit 160.
The rectifier circuit 160 comprises four diode circuits that operate using well known principles to pass the voltage signal VOUT to a power storage unit 180. The voltage VOUT is passed through the first electrical connection 150 and the rectifier circuit 160 to the power storage unit 180. The voltage VOUT is passed through the second electrical connection 170 and the rectifier circuit 160 to the power storage unit 180. The voltage VOUT from the piezoelectric layer 130 is accumulated in the power storage unit 180. The voltage that is accumulated in the power storage unit 180 may subsequently be used to provide an alternative power source (designated Power Output in FIG. 1).
The prior art piezoelectric vibrating energy harvester 100 described above generally has an output performance that produces a high voltage and a low current. It would be desirable to have a vibrating energy harvester that could produce both a high voltage and a high current. It would also be desirable to have a vibrating energy harvester that could generate and store more electrical energy than a piezoelectric vibrating energy harvester can generate and store.